This invention relates to a sighting device for an examination of the eye. It also relates to a sighting method implemented in this device, as well as a system for examining the eye by in vivo tomography equipped with this device.
While examining the eye in general and the retina in particular, unconscious movements of the eye, even during a fixation, can considerably limit the performance of the examination.
Residual movements during a fixation are of three types:                Physiological nystagmus (or tremor): very rapid oscillations (from 40 to 100 Hz), of low amplitude (movement of images of the order of a micron on the retina);        Drift: slow movements (1 μm in a few ms), decorrelated from one eye to the other;        Micro-saccades: very rapid movements (a few hundreds per second), correlated between the eyes, for approximate recentring of the field.        
Experience shows that fixation performances for a given subject are very variable, depending on the subject's state of fatigue, the ambient lighting or the fixation period. It is also known that fixation with both eyes is better than with a single eye.
The addition of a system for compensating movements of the eye may be shown to be very complex, costly, and sometimes incompatible with the existing instrumentation.